1. Field of the Invention
This invention relates to a heat insulating engine formed of ceramic materials.
2. Description of the Prior Art
A conventional heat insulating engine that uses heat insulating members or heat resistant members formed of ceramic material is disclosed in the Japanese Patent Application Laid-Open No. 122765/1984 filed by this inventor. This is briefly explained by referring to FIG. 5. The heat insulating engine 40 described in the above patent application has fitted inside a cast cylinder head 43 a ceramic liner head 41 which has a cylinder liner 42. The liner head 41 consists of a cylinder head inner wall 52 and a cylinder liner upper portion 51, integrally formed in one piece. The cylinder head inner wall 52 is the portion most exposed to hot and high pressure gas during one cycle of engine and also the one through which heat dissipates most. A piston head 44 is formed of silicon nitride with a recess 45 at the center and with a step 46 formed at the lower end circumference to position a piston body 47 and prevent its dislocation. The piston head 44 has a bolt insertion hole in the center recess 45 to secure the piston body 47 thereto. The piston body 47 has at the upper end circumference a step 48 to receive the lower end circumference of the piston head 44. The piston body 47 also has its top center portion raised and the upper surface of the raised portion 49 is placed in contact with the underside of the piston head 44. Then, the piston head 44 and the piston body 47 are held together by bolt 50. The piston head 44 is formed thick and in one piece.
The Japanese Patent Application Laid-Open No. 119892/1986 discloses a heat insulating structure of heat engine in which a hollow portion between metal structure and a ceramic heat insulating wall is filled with a heat convection prevention material such as ceramic fibers and stainless steel fibers. This heat insulating structure of heat engine has a heat reflection plate of heat resistant metal on the inner wall of the hollow portion. The piston of this structure has a piston head formed thick and in one piece, as with the preceding example.
With the above heat insulating engine members, such as piston, that use ceramic materials for heat insulating or heat resistant members, however, it is very difficult to provide a sufficient heat insulating performance. This is because the ceramic member is exposed to high temperatures in the combustion chamber and thus undergoes a heat shock, which gives rise to a problem in terms of strength of the ceramic member. On the other hand, when the thickness of the ceramic member on the wall is increases for better heat insulation, the heat capacity will increase, so that fresh air drawn into the cylinder during the intake stroke receives greater amount of heat from the combustion chamber and therefore is heated to higher temperatures, reducing the air intake efficiency, making it difficult for a sufficient amount of air to be taken in. On the contrary, the heat insulation must be improved during the compression stroke.
In the heat insulating engines described in the foregoing patent applications (Japanese Patent Application Laid-Open Nos. 122765/1984 and 119892/1986), the ceramic piston head is formed with a recess and thus required to have a very large thickness to have a sufficient strength. On the other hand, to improve the air intake efficiency the heat capacity of the piston head must be made as small as possible. The piston head has these two contradicting requirements and therefore has inherent problems similar to those mentioned above.
This is explained below. FIG. 4 is a graph showing the temperature variations of the piston head with the lapse of time during engine operation. In engines with its piston head formed monolithic or in one piece, like the foregoing heat insulating engines, the temperature reduction in the power stroke and exhaust stroke is small and a high temperature state continues, as indicated by a broken line M in the graph of FIG. 4. The temperature in the combustion chamber during the intake stroke is not sufficiently low so that fresh air is not easily drawn into the combustion chamber in sufficient quantity, reducing the air intake efficiency.